1. Field of the Invention
The present invention relates generally to a method and system for configuring and managing channels in a wireless communication system using a multiple access scheme. More particularly, the present invention relates to a method for allocating resources and transmitting data with the allocated resources in a communication system using Orthogonal Frequency Division Multiplexing (OFDM), and a system for managing the same.
2. Description of the Related Art
For mobile communication systems, active research is being conducted on using the OFDM scheme for high-speed data transmission over wired/wireless channels. The OFDM scheme, a technique for transmitting data using multiple carriers, is a type of Multi-Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbols and modulates each of the symbols with a plurality of orthogonal sub-carriers before transmission.
MCM was first used in military high-fidelity (HF) radios in the late 1950s, and the OFDM scheme began to develop in the 1970s. However, there were limitations in the application of MCM and OFDM schemes to a communications system because of the difficulty in implementing orthogonal modulation between multiple carriers. After Weinstein, et al. showed in 1971 that OFDM modulation/demodulation can be efficiently achieved using Discrete Fourier Transform (DFT), the development of the OFDM technology rapidly progressed. In addition, the recent introduction of a method of using a guard interval and inserting a cyclic prefix (CP) in the guard interval has reduced the multi-path delay spread effects.
The OFDM scheme is increasingly applied to digital transmission technologies, examples of which include Digital Audio Broadcasting (DAB), Digital Television, Wireless Local Area Network (WLAN), Wireless Asynchronous Transfer Mode (WATM). The realization of the OFDM scheme is made possible by recent developments of various digital signal processing technologies, including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) which were not previously commonly used due to their high degree of hardware complexity. The OFDM scheme, while similar to conventional Frequency Division Multiplexing (FDM), is characterized by maintaining orthogonality between multiple sub-carriers during transmission, thereby obtaining the optimal transmission efficiency during high-speed data transmission.
In addition, the OFDM scheme can obtain the optimal transmission efficiency during high-speed data transmission because it is robust against multi-path fading. Further, the OFDM scheme, since it overlaps frequency spectra, has high frequency efficiency and is robust against frequency selective fading and multi-path fading. Moreover, the OFDM scheme can reduce inter-symbol interference (ISI) with the use of the guard interval. In addition an equalizer of low hardware complexity can be designed for use with the OFDM scheme. Also the OFDM scheme is robust against impulse noises. Because of the above benefits the OFDM scheme is being actively applied to communication systems.
In wireless communications, deterioration of high-speed, high-quality data service is generally caused by the channel environment. For wireless communications, the channel environment is subject to frequent change due not only to Additive White Gaussian Noise (AWGN), but also to power variations of received signals caused by fading, shadowing, a Doppler effect caused by movement and frequent velocity change of the terminals, and interference caused by signals from other users and multi-path signals. Therefore, in order to support high-speed, high-quality data service, wireless communication systems need to efficiently overcome the foregoing disadvantageous factors.
In the conventional OFDM system, the transmission technologies used for coping with the fading can be roughly classified into two kinds: one is Adaptive Modulation and Coding (AMC) technology and the other is Diversity technology.
First, the AMC technology will be described.
The AMC technology adaptively controls a modulation scheme and a coding scheme according to channel variation of a downlink. Generally, Channel Quality Information (CQI) of the downlink can be detected by a terminal by measuring a Signal-to-Noise Ratio (SNR) of a received signal. That is, the terminal feedback-transmits the CQI of the downlink to a base station over an uplink. The base station estimates a channel state of the downlink depending on the CQI of the downlink fed back from the terminal. The base station controls its modulation scheme and coding scheme according to the estimated channel state.
The AMC technology generally uses high-order modulation and a high coding rate for a good channel state, and low-order modulation and a low coding rate for a bad channel state. The AMC scheme, compared with the conventional scheme based on high-speed power control, increases application capability for time-variable characteristics of the channel, thereby improving average system performance.
FIG. 1 is a diagram illustrating an exemplary AMC operation in a conventional OFDM system.
Referring to FIG. 1, reference numeral 101 denotes one sub-carrier, and reference numeral 102 denotes one OFDM symbol. In FIG. 1, the horizontal axis represents a time axis, and the vertical axis represents a frequency axis. As illustrated in FIG. 1, an OFDM system using the AMC technology generally divides the full frequency band into N sub-carrier groups #1 to #N, and performs an AMC operation per sub-carrier group. Herein, one sub-carrier group will be referred to as “one AMC sub-band.” That is, a sub-carrier group #1 denoted by reference numeral 103 is referred to as an “AMC sub-band #1,” and a sub-carrier group #N denoted by reference numeral 104 is referred to as an “AMC sub-band #N.” In the conventional OFDM system, scheduling is performed in units of a plurality of OFDM symbols as denoted by reference numeral 105.
As described above, the AMC operation in the conventional OFDM system is independently performed per AMC sub-band. Each terminal feeds CQI information for each individual sub-band back to a base station, and the base station performs scheduling on each sub-band depending on the CQI information for each sub-band received from the terminals and transmits user data per sub-band. In the exemplary scheduling process, the base station selects a terminal having the best channel quality for each individual sub-band, and transmits data to the selected terminal, thereby maximizing the system capacity.
In the AMC technology, it is preferable that multiple sub-carriers necessary for transmitting data for one terminal are adjacent to each other. This is because when frequency selectivity occurs in a frequency domain due to the multi-path wireless channel, the adjacent sub-carriers are similar to each other in strength of the channel response, but the sub-carriers spaced apart from each other may be greatly different in the strength of the channel response. That is, because the AMC operation maximizes the system capacity by gathering sub-carriers with a good channel response and transmitting data through them, there is a need for a structure capable of gathering a plurality of adjacent sub-carriers with a good channel response and transmitting data using the gathered sub-carriers.
The AMC technology is suitable for communications traffic transmitted to a particular user. This is because it is not preferable that the channel transmitted to a plurality of users, for example, the broadcast channel or the common control information channel, is adapted to the channel state of a certain user. In addition, the AMC technology is appropriate for transmission of communications traffic which is less susceptible to delay. This is because the AMC technology basically selects the terminals in a good channel state and then transmits data to only the selected terminals, so for delay-sensitive communications traffic, for example, real-time communications traffic such as Voice-over-IP (VoIP) or video meetings, the corresponding user cannot continue to wait until the channel state becomes better. For the users receiving real-time communications traffic service, it is necessary to transmit data to the corresponding users even in the bad channel state in order to guarantee a minimum amount of delay.
Second, the diversity technology will be described.
As described above, the AMC technology is not appropriate for delay-sensitive communications traffic, such as real-time communications traffic that should not be adapted to the channel environment of a specific user, like the broadcast channel and common control channel. However, diversity technology is one of the communication technologies suitable for the transmission of the delay-sensitive communications traffic or communications traffic shared by a plurality of users.
Generally, an amount a wireless channel suffers from a poor channel environment changes in the time domain. Even in the frequency domain, the wireless channel has a good state in one band and a bad state in another band on a repeated basis. In this channel environment, when data transmission cannot be adapted to the channel for a certain user, each terminal receiving the transmitted data inevitably faces the phenomenon in which it sometimes receives the data in a good channel state and sometimes receives the data in a bad channel state. The diversity technology is appropriate for such wireless environments or communications traffic. Therefore, diversity technology aims at allowing the transmission communications traffic to uniformly experience good channels and bad channels, if possible, for the following reasons. If a terminal receives a specific packet in a bad channel state, it will have difficulty in successfully decoding the received packet. In terms of the reception performance, however, if modulation symbols included in one packet include some symbols experiencing bad channels and other symbols experiencing good channels, the terminal can perform packet demodulation using the symbols experiencing the good channels.
FIG. 2 is a diagram illustrating an exemplary method for transmitting user data or common control information using the diversity technology in the conventional OFDM system.
It is assumed in FIG. 2 that downlink data is being transmitted from a base station to three different mobile stations MS1, MS2 and MS3. It can be understood from FIG. 2 that when data is transmitted with the diversity technology, data transmitted to one user is spread over the frequency domain and the time domain. More specifically, data symbols for MS1 transmitted for an OFDM symbol interval denoted by reference numeral 201 occupy three sub-carriers. Typically, it is important that their positions spread over the full band in order to obtain diversity in the frequency domain, and that the specific positions are predefined between the base station and the terminals.
In addition, it can be noted that the symbols transmitted to the MS1 for the OFDM symbol interval 201 are different in position from the symbols transmitted to the MS1 for the OFDM symbol interval denoted by reference numeral 202. This is to change the sub-carriers through which data symbols will be transmitted, for every OFDM symbol or every predetermined transmission unit, in order to maximize the diversity effect in the time domain. This technique is called “frequency hopping,” and most OFDM systems employing diversity technology use a frequency hopping technique.
As described above, the AMC and diversity technologies, used to overcome the fading phenomenon in the OFDM system, are opposite to each other not only in their characteristics, but also in their appropriate communications traffic types. Accordingly, there is a need to operate a communications system such that it appropriately combines the two technologies, rather than using only one of the technologies.